【Qt源码笔记】从WinMain说起

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Qt在各个平台下都是对平台API进行了一些包装。Windows下是对Win32API的封装。如果是Windows平台的GUI Application就一定是从WinMain开始。

不难发现WinMain就在qtmain_win.cpp中。

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extern "C" int APIENTRY WinMain(HINSTANCE, HINSTANCE, LPSTR /*cmdParamarg*/, int /* cmdShow */)
{
    int argc;
    wchar_t **argvW = CommandLineToArgvW(GetCommandLineW(), &argc);
    if (!argvW)
        return -1;
    char **argv = new char *[argc + 1];
    for (int i = 0; i < argc; ++i)
        argv[i] = wideToMulti(CP_ACP, argvW[i]);
    argv[argc] = Q_NULLPTR;
    LocalFree(argvW);
    const int exitCode = main(argc, argv);
    for (int i = 0; i < argc && argv[i]; ++i)
        delete [] argv[i];
    delete [] argv;
    return exitCode;
}

在这里的WinMain仅仅充当一个入口，所有对命令行参数 这些 都交由main来处理。而这个main就是我们在自己的主程序中写的main。

入口找到以后，在Windows下的程序还有一个很重要的东西，那就是消息循环。Win32中经典的PeekMessage()、DispatchMessage()和TranslateMessage()。这些东西在程序中注册的回调函数中被调用，用来处理和解析消息。Qt本身也要依赖这些，只不过在上边进行了一些封装。调到我们自己的程序里看到的就是winEvent()或者是一些QEvent了。

我们写Qt程序的时候，一个很常见的套路是：

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int main(int argv, char **args)
{
    QApplication app(argv, args);
    //todo...
    return app.exec();
}

这个回调函数就是在app.exec()中被注册(准确的说回调函数是由在这个方法中调用的其他方法注册)。不难找到一个叫做qeventdispatcher_win.cpp文件，名字已经很明确了，就是处理Qt事件的。我们会找到一个类QEventDispatcherWin32。可以发现一个processEvents()方法。

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bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
{
    Q_D(QEventDispatcherWin32);

    if (!d->internalHwnd) {
        createInternalHwnd();
        wakeUp(); // trigger a call to sendPostedEvents()
    }
    
    //...
    do {
        DWORD waitRet = 0;
        HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];
        QVarLengthArray<MSG> processedTimers;
        while (!d->interrupt) {
            DWORD nCount = d->winEventNotifierList.count();
            Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);

            MSG msg;
            bool haveMessage;

            if (!(flags & QEventLoop::ExcludeUserInputEvents) && !d->queuedUserInputEvents.isEmpty()) {
                // process queued user input events
                haveMessage = true;
                msg = d->queuedUserInputEvents.takeFirst();
            } else if(!(flags & QEventLoop::ExcludeSocketNotifiers) && !d->queuedSocketEvents.isEmpty()) {
                // process queued socket events
                haveMessage = true;
                msg = d->queuedSocketEvents.takeFirst();
            } else {
                haveMessage = PeekMessage(&msg, 0, 0, 0, PM_REMOVE);
                if (haveMessage) {
                    if ((flags & QEventLoop::ExcludeUserInputEvents)
                        && ((msg.message >= WM_KEYFIRST
                             && msg.message <= WM_KEYLAST)
                            || (msg.message >= WM_MOUSEFIRST
                                && msg.message <= WM_MOUSELAST)
                            || msg.message == WM_MOUSEWHEEL
                            || msg.message == WM_MOUSEHWHEEL
                            || msg.message == WM_TOUCH
#ifndef QT_NO_GESTURES
                            || msg.message == WM_GESTURE
                            || msg.message == WM_GESTURENOTIFY
#endif
                            || msg.message == WM_CLOSE)) {
                        // queue user input events for later processing
                        d->queuedUserInputEvents.append(msg);
                        continue;
                    }
                    //...
                }
            }
            //...
            if (haveMessage) {
                //...
                if (!filterNativeEvent(QByteArrayLiteral("windows_generic_MSG"), &msg, 0)) {
                    TranslateMessage(&msg);
                    DispatchMessage(&msg);
                }
            } 
            //...
            retVal = true;
        }

        // still nothing - wait for message or signalled objects
        canWait = (!retVal
                   && !d->interrupt
                   && (flags & QEventLoop::WaitForMoreEvents));
        if (canWait) {
            DWORD nCount = d->winEventNotifierList.count();
            Q_ASSERT(nCount < MAXIMUM_WAIT_OBJECTS - 1);
            for (int i=0; i<(int)nCount; i++)
                pHandles[i] = d->winEventNotifierList.at(i)->handle();

            emit aboutToBlock();
            waitRet = MsgWaitForMultipleObjectsEx(nCount, pHandles, INFINITE, QS_ALLINPUT, MWMO_ALERTABLE | MWMO_INPUTAVAILABLE);
            emit awake();
            if (waitRet - WAIT_OBJECT_0 < nCount) {
                d->activateEventNotifier(d->winEventNotifierList.at(waitRet - WAIT_OBJECT_0));
                retVal = true;
            }
        }
    } while (canWait);

    // ...
    return retVal;
}

代码比较长，省略了一些暂时不关注的，在这里我们可以看到我们最熟悉的Win32的消息枚举和方法。现在问题来了DispatchMessage()以后，程序的调用会走到我们注册的回调函数，由我们自己来处理消息。所以要找到这个回调。

Qt的这个回调函数是qt_internal_proc()。那下一个问题就是在哪里注册的这个回调函数。
这个可以回顾一下Win32程序的一般套路：

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WNDCLASSEX wc;
HWND hwnd;
MSG Msg;
//Step 1: Registering the Window Class
wc.cbSize        = sizeof(WNDCLASSEX);
wc.style         = 0;
wc.lpfnWndProc   = WndProc;
wc.cbClsExtra    = 0;
wc.cbWndExtra    = 0;
wc.hInstance     = hInstance;
wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wc.lpszMenuName  = NULL;
wc.lpszClassName = g_szClassName;
wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);
if(!RegisterClassEx(&wc))
{
    MessageBox(NULL, "Window Registration Failed!", "Error!",
        MB_ICONEXCLAMATION | MB_OK);
    return 0;
}

如果要写一个Win32的程序，都要先注册一个Windows Class,就是在lpfnWndProc中指明我们的回调方法。再回过头去看processEvents()方法中createInternalHwnd()的调用:

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void QEventDispatcherWin32::createInternalHwnd()
{
    Q_D(QEventDispatcherWin32);

    if (d->internalHwnd)
        return;
    d->internalHwnd = qt_create_internal_window(this);

    installMessageHook();

    // start all normal timers
    for (int i = 0; i < d->timerVec.count(); ++i)
        d->registerTimer(d->timerVec.at(i));
}

可以看到一个名叫qt_create_internal_window()的方法，顾名思义。在此方法中会获取一个QWindowsMessageWindowClassContext,看一下他的构造函数，一目了然:

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QWindowsMessageWindowClassContext::QWindowsMessageWindowClassContext()
    : atom(0), className(0)
{
    // make sure that multiple Qt's can coexist in the same process
    const QString qClassName = QStringLiteral("QEventDispatcherWin32_Internal_Widget")
        + QString::number(quintptr(qt_internal_proc));
    className = new wchar_t[qClassName.size() + 1];
    qClassName.toWCharArray(className);
    className[qClassName.size()] = 0;

    WNDCLASS wc;
    wc.style = 0;
    wc.lpfnWndProc = qt_internal_proc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = qWinAppInst();
    wc.hIcon = 0;
    wc.hCursor = 0;
    wc.hbrBackground = 0;
    wc.lpszMenuName = NULL;
    wc.lpszClassName = className;
    atom = RegisterClass(&wc);
    if (!atom) {
        qErrnoWarning("%s RegisterClass() failed", qPrintable(qClassName));
        delete [] className;
        className = 0;
    }
}

不难发现回调函数qt_internal_proc()就是在这里注册的。

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LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
{
    if (message == WM_NCCREATE)
        return true;

    MSG msg;
    msg.hwnd = hwnd;
    msg.message = message;
    msg.wParam = wp;
    msg.lParam = lp;
    QAbstractEventDispatcher* dispatcher = QAbstractEventDispatcher::instance();
    long result;
    if (!dispatcher) {
        if (message == WM_TIMER)
            KillTimer(hwnd, wp);
        return 0;
    } else if (dispatcher->filterNativeEvent(QByteArrayLiteral("windows_dispatcher_MSG"), &msg, &result)) {
        return result;
    }
    //...
    return result;
}

Qt就是这样将Win32的调用包装成了自己的调用。